Alumina is a widely used material in various fields mainly due to the ability of preparing it with a wide variety of morphological properties. Specifically, it can be prepared with surface areas of up to several hundred square meters per gram and pore size distributions with diameters ranging from a few nanometers to several hundred nanometers. One application of particular importance is the use of alumina as a base material in making heterogenous catalysts, particularly for petroleum refining. In this application, alumina powders are shaped into various forms such as spheres, pellets or extrudates. The properties that are considered to be important for this application are the surface area (SA), the mechanical strength (MS) and the pore size distribution (PSD) of the formed alumina. In general, the three properties are interrelated in that alumina with large pores normally has low surface area and poor mechanical strength.
During recent years, it has become increasingly important to design and manufacture alumina catalyst support materials and catalysts having wide pores while maintaining reasonably high mechanical strength. The alumina materials are particularly important for processing petroleum heavy residues, coal liquids, or other feedstocks. Such feedstocks contain large asphaltenic molecules that are chemically resistant to cracking and hydrotreatment, and high percentages of foulant metals mostly associated with the asphaltenic molecules that deposit on the surface of the catalyst support. Thus if the catalyst has small pores, the tendency of the foulant metals is to deposit on the outer surface of the pellet due to the small size of the pore mouth. The continuous deposition of the foulant metals on the pellet surface eventually blocks the entry to the inner pores and leads to the deactivation of the catalyst. Hence, intensive efforts have been made during the past two decades to make alumina extrudates with larger pore diameters to overcome this problem.
Accordingly, there is a need for a sufficiently flexible process that can be used to make extrudates having a narrow distribution of pore sizes, with the desired average pore diameter, and at the same time maintaining sufficient strength to withstand the vigors of petroleum processing. It is also desirable to provide a process for making catalysts supports with a narrow distribution of large diameter pores and relatively high strengths for hydrotreating heavy petroleum residues.
In the past, there have been numerous approaches for producing alumina catalysts and supports with controlled pore sizes. For example, D. L. Trimm and A. Stanislaus in "The Control of Pore Size in Alumina Catalyst Supports: A Review," Applied Catalysis, 21 (1986) 215-238 discusses the state of the art at that time. As pointed out therein, it is often necessary to control both large and small pores and there are a number of approaches for doing so. For example, large pores generally originate from inter-particle spaces and their size may be controlled by mixing alumina powder with graphite, pelleting and then removing the graphite. Small pores originate from spaces within the particle caused for example by removal of water between the crystal planes.
The Trimm and Stanislaus article also points out that the control of pore size in alumina, i.e., the larger pore sizes can be accomplished by control of particle agglomeration, size and shape of the agglomerates and that polymer additives may increase macroporosity as a result of space filling.
Several methods have been utilized in the literature to vary the pore structure of extrudates. Such methods include varying the microcrystal structure and agglomerate size of the alumina, the type of alumina salt used, the precipitating agent, the operating condition during precipitation such as temperature, pH and aging of the solution and the method and conditions of drying. Other factors affecting pore structure include pelleting or extruding conditions, such as pressure and temperature, particularly with respect to macropores, i.e., pores with diameters greater than 50 nm (500 Angestrom Units) and the effect of pore forming additives such as polymers and carbon powders used to generate macropores. Such additives have been added to an alumina during kneading and then when the extrudates or pellets are calcined, the organic material decomposes or burns forming water and then volatile oxides, thus leaving voids in place of the decomposed substances.
It has now been found that substrates and catalysts may be produced by the processes described herein with pore structure tailored to a specific application, and that such substrates can be produced with narrow monomodal pore-size distribution having different average pore diameters ranging from small diameters of around 50 angstrom to as much as 1000 angstrom units. Substrates with narrow bimodal pore size distribution with one set of small pores having relatively small average diameters and large pores having large average diameters can also be produced.
It has also been found that substrates having wide pores produced by the presently disclosed process have relatively high strength as compared to the wide pore substrates reported in the prior art literature.